The present invention relates to a large scale movie display system with multiple gray levels which comprises specific circuits, e.g. negative feedback or image current circuits, to control the brightness of the LEDs in proportion to the analog signals produced by the image information. The overall circuit can be used to carry out presetly, or real-timely the programmably controlled image display through a compact personal computer or microprocessor for attractive dynamic image advertising and displaying effects.
At present, electronic displaying boards are becoming more and more popular. In general, a conventional television set may well serve the displaying functions. However, when used in large scale displaying boards, the functions provided by such conventional television set would be worse than the electronic displaying boards. In particular, the electronic displaying boards are better at providing a programmably controlled moving image display for advertising, announcing or indicating purposes, such as for diversified uses in stock markets, airports and communication stations for displaying time schedules and information, and as the large scale display for use in stadiums.
These conventional electronic displaying boards are composed of array of visible light dots which, in the past, are composed by a plurality of incandescent bulbs. The type of electronic displaying board has one disadvantage that the board would remain a residual image of the preceding image when changed to a new image since the heating time constant of each of the bulbs is too long. This condition results in that pictures displayed can not be changed fast. Moreover, the current consumption and power supplied for the above device are so great that upon driving, the current must be first amplified by power elements. Further, owing to the high rate to damage of the bulbs and frequent replacements, it is time- and labor-consuming in maintenance.
The recent discovered electronic displaying boards composed of the LED array have overcome the disadvantages resulting from the array of incandescent bulbs. The electronic displaying board with LEDs has the advantages of a longer usage life (more than ten years), reduced dimensions, and smaller operating voltage (1.5-2.4 V) and current (5-20 mA). Further, it can provide red, yellow, and green colors, for displaying; the spectra thereof narrower being such that the visual sensitivity to the eyes is stronger. Therefore, this type of electronic displaying board is becoming more and more popular for advertising and demonstration purposes.
The LEDs are one type of the solid state electronic elements and thus, the small panel of LED array (5.times.7 or 8.times.8) can be quite easily manufactured and packaged by means of automatic machines. Further, the diodes have a reverse blocking characteristic such that they can form a bridged array, as shown in the circuit of FIG. 11, which is a more commonly used circuit at the present. In the circuit, scanning function is achieved via synchronous multiplexing in the X and Y directions and thereby certain programmed stationary graphic displays can be executed.
However, since the present large scale electronic LED displaying boards can only provide two gray levels, that is, either full bright or full dark, they are capable only of graphic display. When used for displaying images, the images look like cartoon pictures without stereo feeling in different gray levels. This is due to the strong non-linear current-voltage characteristic of the LEDs, which is, therefore, rather difficult to have the current-voltage characteristic curve of the LEDs linearized to produce different brightness. If controlling a small LED array, such as 4.times.4, in a half tone manner, then 16 gray levels of the display can be obtained. However, the number of arrays in the displaying board will thus be reduced so as to affect the resolution of the image.
In addition, with respect to a displaying board with single dot LED gray level, the variation in mean brightness of the display can be controlled by means of controlling the operating time of fast flashing pulses. Under this condition, if the flashing frequency of the pulses is higher than the frequency for persistence of vision, then, as viewed to human eyes, the variation in brightness will be in proportion to the operating time of these pulses. This manner can be readily accomplished in a displaying board with small LED array. However, in large scale LED displaying boards, such as displaying screen in sports field, there are drawbacks difficult to overcome in design. This will be described by way of the following example:
Displaying screen: N.times.N array
Specifications of a single LED: 1.8V, 20mA
Frame rate: 30 frames/sec (number of frames per second)
Pixel dwell time: T=1/(30.times.N.sup.2)
To maintain equal visual brightness, the transient trigger current I for each of the LEDs should meet the following equation: EQU I.times.T=20mA.times.(1/30)
that is, EQU I=20mA.times.(1/30).times.30.times.N.sup.2 =20 mA.times.N.sup.2
When the frame is in the form of an array of 256.times.256 pixels,
then I=20 mA.times.256.times.256=20.times.64=1280 A
which is found to be impossible.
If LEDs of 5 mA are used, the transient trigger current for each LED must be as high as 320 A which is still impossible because the presently available LEDs have a maximal transient current lower than 100 A and requires a driving voltage higher than 100 V.
Moreover, as well as known, each LED itself has an inherent serial resistance of about 1 ohm, which is formed by the chip resistance between the P-N interface and the substrate of the LED, and the contact resistance between the packed silver glue and the chip surface during packing. When the LED is supplied by continuous direct current, the current flowing therethrough is about 20 mA as described in the above specification, and thus both the voltage drop thereof and the power consumption are relatively low. However, when the LED is activated by means of pulsed voltage source for transient switching on, the LED would have a large voltage drop of about 1000 V due to its enormous transient current, which is infeasible. Even in the case where such high voltage driving is possible, the power consumption of the LED would be thousand times that required for an ideal LED. Under such condition, the light emitting efficiency of the LED would be sharply decreased due to the temperature rise at the interface or the LED would be damaged due to such high temperature rise.
In brief, gray level control of the LED brightness by means of controlling the width of the operating time of the , transient flashing pulse will become infeasible in design as the dimensions (N x N) of the display increase. This is the reason why the large scale LED image (not graphic) displays are not yet introduced in the market.